Stare at the eye of the red parrot while you count slowly to
20, then look immediately at one spot in the empty bird cage.
The faint, ghostly image of a bluegreen bird will appear in
the cage.

Try the same thing with the green cardinal.
A faint magenta bird will appear in the cage.

The ghostly birds you see here are called afterimages. An afterimage
is an image that stays with you even after you have stopped
looking at an object. The back of your eye is lined with light
sensitive cells, called cones, which are sensitive to certain
colors of light. When you stare at the red bird, your red-sensitive
cones adapt to the light and lose their sensitivity. When you
shift your gaze to the white background of the bird cage, you
see white (minus red) where the red-sensitive cells have
become adapted. White light minus red light is blue-green light.
That's why the afterimage you see is blue-green and in the shape
of a parrot. The same thing happens when you stare at the green
bird, but this time it's the green-sensitive cones that adapt.
White minus green light is magenta light, so you see the afterimage
as a magenta cardinal.

Here's another afterimage:
Look at the flag for about 15 seconds, then scroll down to the
white space provided below. Notice how the ghostly image of
the familar "stars and stripes" appears. Like Bird in a Cage,
this afterimage occurs because red, white and blue are the complementary
colors of cyan,black and yellow.

Originally rendered in 1994 by Cija Briegleb
and Zach Waller, students at San Francisco State University
department of Information Arts and Conceptual Design. Contact
zwaller@sfsuvax1.sfsu.edu for comments.